Protective relays are used at various points in power transmission and distribution systems to identify faults and to actuate a circuit breaker covering the portion of the system encompassing the fault, until the fault is cleared. In recent years, computer-type relays using microprocessors have become increasingly popular and are now widely used. In such relays, the voltage and current signals on the power line are sampled. These signal samples are then applied through transformers and low pass filters and then a multiplexer. The multiplexed signals are then applied to an A/D converter and from there are applied to a microprocessor, which compares various actual characteristics of the signals against selected standard values of those characteristics to ascertain fault conditions. The general operation of such relays is well known in the art.
While such relays are ordinarily extremely reliable, certain portions of the relay, such as for instance the A/D converter, can malfunction in various ways. When the resulting signals are processed by the microprocessor, they could erroneously appear to be indicative of a fault condition on the power system. This could result in an incorrect trip of the circuit breaker, interrupting power to a portion of the power system, which is very undesirable.
In the present invention, two possible sources of erroneous data are identified and the erroneous signals are prevented from producing a trip output signal which in turn would actuate a circuit breaker.